The glasses most used for the encapsulation of germanium and silicon crystal diodes have been Corning 0120 glass and Corning 8870 glass, the compositions of which are tabulated below in Table I. Table I also reports the softening point (S.P.), coefficient of thermal expansion (Exp.) over the range 0.degree.-300.degree. C. (.times.10.sup.-7 /.degree. C.), and the viscosity (Vis.) of the glass at its liquidus in poises, as determined in accordance with techniques conventional in the glass art. Sovirel glass D0030, also recorded in Table I, closely approximates the composition of Corning 8870 glass and exhibits similar physical properties. Those glasses seal well to the Dumet metal leads customarily employed with such diodes. Dumet consists of a nickel-iron core enclosed in a copper sheath and demonstrates a coefficient of thermal expansion over the range of 0.degree.-300.degree. C. of about 80-95 .times. 10.sup.-7 /.degree. C.
TABLE I ______________________________________ 0120 8870 D0030 ______________________________________ SiO.sub.2 55.0 35 34.5 Al.sub.2 O.sub.3 1.45 -- -- Na.sub.2 O 3.60 -- -- K.sub.2 O 8.70 5.5 5.7 PbO 29.0 59 59.4 As.sub.2 O.sub.3 0.25 0.5 0.4 S.P. 630.degree. C. 580.degree. C. 559.degree. C. Exp. 89.5 91.0 93.0 Vis. &gt;10.6 350,000 200,000 ______________________________________
Recent research specifically directed to the encapsulation of germaniun and silicon diodes has been conducted to develop glasses which have lower softening points to thereby reduce the time required for sealing and, in so doing, to increase output. The glass should also be substantially free from Li.sub.2 O and Na.sub.2 O to avoid any contamination of the germanium or silicon crystal diodes. Furthermore, the viscosity of the glass at the liquidus should be at least 20,000 poises to enable the drawing of tubing by means of the Vello process. The glass enclosures used for encapsulating semiconductor devices are customarily cut from tubing.